There were many radical movements and ideologies clashing during those times.
There was geopolitical instability and fast technology/society changes.
Its easy to judge him today. But times were different back then.
Despite all the internal/external problems and pressure he kept the country in one piece;
Under his rule ussr went through rapid industrialization and became a superpower.
Maybe many russians perceive their country as weak today and that makes them think of stalin positively?